Upconversion nanoparticles: influence of the host lattices on crystallographic and luminescent properties

Ytterbium/erbium co-doped NaYF 4 , NaGdF 4 , CaF 2 , and LaF 3 upconversion nanoparticles (UCNPs) were synthesized by hydrothermal process. X-ray diffraction pattern (XRD), Thermogravimetric analysis (TGA), Fourier transforms infrared (FTIR), UV/visible, and photoluminescence spectroscopic techniques were employed to determine the crystal structure, grain size, phase pureness, thermal stability, surface functional groups, optical properties, and UC luminescence properties. CaF 2 host lattice presented a cubic face-centered structure, whereas NaYF 4 , NaGdF 4, and LaF 3 host matrices exhibited a hexagonal phase of the materials. The lattice parameters were calculated from the reflection peak positions. NaYF 4 host material exhibited higher thermal stability, and LaF 3 was the least among the presented host matrices. FTIR spectra display the presence of carboxylic moieties which assist in the formation of colloidal solution. Absorption spectra illustrated excellent colloidal dispersibility and optical properties of the UCNPs in an aqueous solvent. Energy bandgap values were estimated from the UV/visible spectra graph plotted between ( αhν ) 2 vs photon energy ( hν ) are to be 5.64, 5.50, 5.82, and 5.71 eV for the NaYF 4 :Yb/Er, NaGdF 4 :Yb/Er, CaF 2 :Yb/Er, and LaF 3 :Yb/Er UCNPs, respectively. UC luminescence spectra were recorded to investigate the impact of the host lattices on the luminescence intensities. In the presented results, NaYF 4 host matrix produces the highest emission intensity, and the LaF 3 host lattice was the least among the observed results. The findings of these non-functionalized UCNPs will be highly useful in luminescent-based biomedical applications.